First of all, don't get confused by
mg and
ml.
Mg is the unit of strength of the nicotine.
Ml is the unit of liquid volume.
Tom has a 60 ml bottle of 12mg nicotine e-liquid.
Yes, more nic can be added to your current bottle of e-liquid. It won't kill you, but too much nic can cause symptoms of headache, jitters, or nausea if its too much.
I can't explain why, but many e-liquid manufacturers aren't making liquid over 6mg nic. Its a recent trend. Your vape shop may be like this. However, there are still some manufacturers who are making e-liquid at 12mg or 18mg pre-mixed. You may have to search for
online sources for higher nic e-liquids. Try My Vapor Store. They have literally dozens of different e-liquid manufacturers; some of them should be making higher nic e-liquid.
myvaporstore | Online Vape shop - Vape Mods, Eliquid and Ecigs
You can also look into "nic salts" e-liquid, which has much higher levels of nicotine than "free base nic" e-liquid.
Nic salt question